The present invention relates to safety devices for lifting mechanisms and more particularly to emergency stop mechanisms for lifting mechanisms using winch-powered cables or lines. Such lifting devices are well known and in common use in elevators, in dumbwaiters and in scissors mechanisms for raising industrial or agricultural equipment.
Such lifting mechanisms often consist of a winch-operated cable or line, a pulley system and a load bearing member. In the case of an elevator, the load bearing member is a vertically translating platform or is the payload itself. The load bearing member is a sliding trolley in the case of agricultural grain elevators or grain augers. Frequently, the failure of any component of the lifting mechanisms will result in a loss of tension in the cable and, therefore, in an absence of lifting force to counteract the force of gravity. The outcome of such a failure may be an undesired rapid downward movement of the payload.
Strengthening some of the individual components of the lifting mechanism may increase the reliable operation of the system and greatly reduce the chance of mechanical failure, but there are practical limitations of cost and size to such strengthening. Furthermore, the reliable operation of the lifting mechanism can be significantly affected only if all of its components are strengthened.
Providing two complete and redundant lifting mechanisms is another alternative, but is an extremely costly one. Furthermore, repeated use of the two lifting mechanisms places the same loads on the backup mechanism as on the primary mechanism and therefore causes similar wear on components of both mechanisms. When one of such mechanisms fails from age, misuse or faulty maintenance, the other mechanism will probably also be in a weakened state. The increased load suddenly placed on the surviving mechanism could cause it to fail also, thereby releasing the payload.
Many safety devices have been designed to prevent the free fall of the payload of lifting mechanisms in the event of such a failure. U.S. Pat. Nos. 1,043,323 (issued Nov. 5, 1912), 3,273,671 (issued Sept. 20, 1966) and 4,106,753 (issued Aug. 15, 1978), for example, disclose examples of safety mechanisms for elevators. In each of these examples there is a secondary cable and there is a clutch normally restrained by tension in the cable of the lifting mechanism. The clutch grips the secondary cable in the event of a drop in tension in the winding line. Each of these examples works well for the type of lifting mechanism for which it is designed, but is needlessly expensive and complex for many applications.
An object of the present invention, therefore, is to provide reliable safety device for use in cooperation with an active lifting mechanism, which device will secure the payload in a raised position in the event of a failure of the primary lifting system. It is also an object of the present invention to provide safety device of this type which is composed of relatively few components which are inexpensive to manufacture and assemble. Finally, it is an object of the present invention to provide a safety device of this type which is easy to disassemble for repair and maintenance.